The effects of acoustic obstruction from rib and other bones have long been a challenge to researchers in high intensity focused ultrasound (HIFU). In several non-invasive surgery applications, such as treatment for liver and pancreatic cancer, it is often the case that the available acoustic windows are partially blocked by the ribs, which can substantially decrease the ultrasound energy delivery to the focal target and may overheat overlying tissues due to the highly absorptive nature of bones. Furthermore, ribs can cause significant field aberration by introducing secondary lobes in the focal profile which can result in undesired collateral damage.
To overcome these issues, significant efforts have been dedicated to develop aberration correction algorithms in order to spare the ribs and improve beam forming. Theoretical studies on the application of virtual phased arrays to sonicate between the rib bones have been conducted; a physically segmented transducer design was also proposed to prevent sonication to the ribs by aligning active elements with the intercostal gaps. More recently, adaptive focal optimization algorithms for transcostal therapy have been developed, depending on the presence of a point source or an identifiable acoustic spot at the desired focus. More sophisticated non-invasive approaches using ultrasound scanning and time-reversal to identify the ribs require transducers with transmit and receive capabilities. Other non-invasive methods explored involve the use of CT or MRI to image the rib obstructions and selectively deactivate elements shadowed by the rib bones.
A major challenge facing trans-thoracic ablation using ultrasound is to overcome the rib obstruction. For example, skin burns and subcostal edema have been reported in clinical HIFU liver ablation cases. For transthoracic ablation of the liver using HIFU, ribs in the ultrasound pathway cause periodic blockage of ultrasound, resulting in a significantly decreased main lobe and increased grating lobes. Moreover, due to the high ultrasound absorption coefficient of bone and reflection effects at the bone-tissue interface, overheating of ribs and surrounding tissue often results in unwanted tissue damage. Phased arrays and aberration correction algorithms have been developed to switch off the elements blocked by the ribs to reduce overheating to the ribs and associated tissue. Even with these improvements, grating lobes may still remain producing undesired heating and collateral damage.
Similarly to trans-thoracic ultrasound ablation, transcranial ultrasound therapy is also very challenging, as the highly aberrating and attenuating effects introduced by the skull can severely distort the therapeutic focus and limit the effectiveness of the treatment. To counter those issues, HIFU systems use non-invasive CT or MR imaging technology to correct for the acoustic aberration effects from the skull (refs) or other sophisticated correction algorithms such as combination of time-reversal method and bubble signature (ref). One of the main challenges in thermal HIFU for transcranial therapy is the need to avoid undesired skull overheating effects, which limit the amount of ultrasound power that can be applied through the skull, even when active cooling is performed on the scalp, potentially reducing the effectiveness of the treatment.
The use of aberration correction algorithms has made it possible to perform noninvasive ultrasound therapy through bone obstruction. However, the difficulties in implementing these correction algorithms is the need for phased arrays and the added complexity of the associated electronics, imaging equipment, and computation. Furthermore, when bone obstacles are involved, secondary lobes may still be present in the focal profile even after correction algorithms are applied due to the periodic ultrasound blockage pattern caused by the bones. Even though a lesion may be generated without overheating the overlying bones using the correction algorithms, the treatment precision could still be poor due to the collateral damage resulted from the increased secondary lobes.